Syntheses of non-aromatic medium and large rings synthesized via phenylnitrenium ions

Abstract

We describe the preparation of m- and p-substituted phenyl azides which, on treatment with trifluoromethanesulfonic acid in chloroform (and only in one case, after adding trifluoroacetic acid) at 0°C, gives rise to the intermediate phenylnitrenium ions that undergo intramolecular cyclization to give six-, eight-membered carbocycles, and ten-membered heterocycles. Intramolecular cyclization of 1-(4-azidophenyl)-4-phenylbutane (3b) gives direct access to the 1,2,3,4-tetrahydronaphthalene lignan scaffold with a good yield. When the same reaction is carried out on 1-(3-azidophenyl)-4-phenylbutane (3a), the meta isomer of 3b, the 3-aminodibenzo[a,c]cyclooctadiene is obtained with a modest yield. When an ethoxycarbonyl group is introduced at position two of the butene chain [16a, as an E/Z mixture (1/4)], the ethyl 3-aminobenzo[a,c]octatriene carboxylate was the major compound, and the 10-membered heterocycle the minor one, both derived from (E)-16a. Finally, when a methyl group is located at the para position of the azido group [(E)-16b], cyclization involves the carbon atom ortho to the nitrogen atom and the ethyl 4-methyl-1-tosylaminobenzo[a,c]octatriene carboxylate is the only compound obtained, after treatment with tosyl chloride. With all these structural changes, we have switched over from the formation of mixtures of compounds to the regioselective formation of the target molecule, suggesting the corresponding mechanism of reaction and expanding the knowledge of this type of reaction.